The present invention relates to a machining apparatus and a machining method for machining a workpiece by rotating a tool and the workpiece relatively and, more particularly, to an apparatus and a method for rotating and revolving either the tool or the workpiece.
In a so-called contouring operation or a machining operation, a cutting tool having a cutting blade is revolved around a work material (workpiece) while the cutting tool is rotated on its own axis. This machining method is typically exemplified by an end milling operation, an example of which is described in Japanese Patent Laid-Open No. 63-212442 (JP-A-63-212442). According to the contouring method described in this publication, before the workpiece is machined by rotating and revolving an end mill, a turning diameter of an edge of the rotating and revolving end mill is measured with a laser beam, and a tool diameter of the end mill is compensated by means of tool diameter compensation function of a numerical control unit on the basis of the result of the aforementioned measurement. In an actual cutting operation, on the other hand, the end mill is at first inserted in the center of a bore to be cut and moved therefrom to a position where the edge of the end mill contacts with the inner surface of the hole to be cut, more specifically, to a position corresponding to a machining diameter, and thereafter moved along the inner face of the bore to be cut. In short, the end mill is revolved by rotating the end mill, as mounted on a spindle of a machine tool, on its own axis and by moving the spindle in a circular motion in a so-called X-Y plane.
In addition to the aforementioned apparatus, there is known an another type of apparatus using a mechanism of a polar coordinate system. More specifically, on a leading end of a main arm, which is rotated or given an oscillatory rocking motion by a motor, there are mounted a motor for the spindle and a tool shaft arm. On a leading end of the tool shaft arm, there is mounted a tool shaft which is rotated by the motor for the spindle. In this kind of apparatus, the tool shaft is moved in a circular motion, that is, revolved either by giving the main arm associated with the tool shaft arm a rocking motion or by rotating the main arm. Radius of revolution of the tool shaft are changed by varying relative angles of the tool shaft arm to the main arm.
There is known a still another type of apparatus which is constructed such that a tool shaft is so mounted on a first axis as to move in the radial direction thereof. This tool shaft is rotated with the first axis, resulting in the tool shaft revolving. The revolution radius of the tool shaft is changed by varying positions of the tool shaft in the radial direction of the first axis.
In the aforementioned contouring operation, a relative speed of the cutting blade to the work material, that is, a cutting speed corresponds to the sum of a circumferential speed of the cutting blade moving in response to rotation of the tool and a speed of a circular motion performed by the cutting blade moving in response to revolution of the tool. According to the aforementioned conventional contouring operation, however, the tool is revolved by giving the spindle having the tool mounted thereon a circular motion, so that the speed of revolution is far lower than the speed of rotation. Therefore, the cutting speed is substantially determined by the speed of rotation of the tool, that is, the number of rotation of the spindle.
In a conventional apparatus using a mechanism of the so-called X-Y coordinate system, the tool is revolved by coordinating the feeds of the spindle in the bisectional directions in the X-Y plane. This restriction of control makes it difficult to revolve the machine tool at a higher speed. When the spindle is moved in the bisectional direction, moreover, a spindle head is vertically moved along a column, and simultaneously with this, the column is horizontally moved on a bed. Thus, the members to be moved have so large mass that the spindle cannot be revolved at a higher speed.
On the other hand, the same problem may occur in an apparatus employing polar coordinate system, in which a reciprocating motion of the main arm is transmitted via its leading end to the tool shaft arm to cause the tool shaft to revolve. Specifically, the main arm is reciprocated with a motor imparting a reciprocating motion to the tool shaft arm and a motor rotating the tool shaft, and the tool shaft is revolved by driving these two motors cooperatively, as in the aforementioned apparatus employing the X-Y coordinate system. Therefore, it is difficult to increase the speed of revolution of the tool shaft.
In still another type of apparatus, too, the tool shaft is revolved with a motor for rotating the tool shaft, so that the overall mass to be revolved is increased, which makes it difficult to revolve the tool shaft at a higher speed. In an apparatus constructed to revolve the tool shaft, the motor for rotating the tool shaft changes its position according to a change of revolution radius of the tool shaft, so that a position of the center of the gravity of a rotating member as a whole changes. As the speed of revolution of the tool shaft is raised, therefore, its vibration may be increased.
In the prior art, in short, it is impossible to increase the speed of revolution of the tool shaft or the cutting tool mounted thereon, and a ratio of revolution speed of the spindle to the cutting speed is not more than 2 to 3%, so that the feed rate per revolution of the tool is lowered. To enhance the working efficiency, therefore, the working operation has to be performed by increasing the speed of rotation of the tool or increasing the width of the cutting blade.
If the cutting width is increased, however, the area where chips contact with the cutting blade is broadened, so that friction and heat is remarkably generated, thereby shortening the tool life. This results in the disadvantage that the working efficiency is greatly restricted. The aforementioned contouring work is the so-called intermittent cutting operation, in which the cutting blade bites into the work material repeatedly. In the method of the prior art as described above, on the other hand, the tool is revolved at a low speed and at a small feed rate, that is, the tool is rotated on its own axis at a high speed, thus increasing the number of times that the cutting blade bites into the work material repeatedly. When this cutting operation is performed, in other words, many wide and long chips are produced. Therefore, an impact force more frequently acts on the cutting blade. Moreover, this impact force is strong, so that the possibility of shortening the tool life is increased, which restricts the working efficiency.
The present invention has been made in consideration of the actual state of art as described above, and has an object to provide an apparatus and a method capable of improving a working efficiency in a machining operation performed by rotating a tool and a workpiece relatively to each other.
Another object of the invention is to provide an apparatus and a method capable of improving a working efficiency and a working accuracy by making it possible to vary a ratio between a number of rotation of the tool or the workpiece and a number of revolution of the tool or the workpiece.
Still another object of the invention is to provide a machining apparatus capable of raising a speed of revolution of the tool or the workpiece at a high level.
Another object of the invention is to provide a machining apparatus capable of changing a radius of revolution of the tool or the workpiece while rotating and revolving the tool or the workpiece.
Another object of the invention is to provide an apparatus and a method capable of improving the working efficiency without shortening the life of the tool.
The machining apparatus of the invention is provided with a spindle rotating while mounting a tool or a workpiece and a revolution shaft rotating on its center axis, and the spindle is rotatably held by the revolution shaft at a position offset from the center axis of the revolution shaft. This machining apparatus is also provided with a rotation driving mechanism for transmitting a power through a rotating member rotating on a center axis of rotation of the revolution shaft or a member rotating integrally with the revolution shaft to the spindle to rotate the same, while rotating the revolution shaft and the spindle at different speeds respectively.
According to the machining apparatus of the invention, therefore, when the spindle and the revolution shaft are rotated on their respective center axes, the tool or the workpiece mounted on a leading end of the spindle is rotated and revolved. By independently rotating the revolution shaft, in this case, the tool or the workpiece mounted on the spindle is revolved. Thus, the speed of revolution is not restricted, so that it is possible to increase a ratio of speed by revolution in the working speed, that is, a relative feeding speed or a feed rate between the tool and the workpiece.
According to the machining apparatus of the invention, moreover, there is provided a motor for revolution having a base portion on which the rotation driving mechanism is fixed, a transmission mechanism for revolution which transmits motive power from the motor for revolution to the revolution shaft, a motor for the spindle fixed on the base portion and a transmission mechanism for the spindle which transmits motive power from the motor for the spindle to the spindle.
With this construction, either of these two motors are fixed, and hence the total mass of rotating elements is decreased, thereby raising the speed of revolution of the spindle at a high level. The transmission mechanism for the spindle is constructed such that torque is transmitted by a roller, thereby preventing fluctuation in rotation of the spindle to allow working operations having a high accuracy.
According to the machining apparatus of the invention, in the revolution shaft, there is arranged an eccentric shaft rotating on an axis eccentric to the center axis of the revolution shaft. At the position eccentric to the center axis of the eccentric shaft, there is rotatably held the spindle.
With this construction, the eccentricity of the tool or the workpiece to the revolution shaft, that is, the revolution radius of the tool or the workpiece is changed by rotating the eccentric shaft. This makes it possible to voluntarily change a relative feed rate of the tool and the workpiece or working radius. By rotating the eccentric shaft during the working operation, moreover, working operations such as taper boring or recessing operation can be performed.
The apparatus of the invention may be provided with a revolution radius changing mechanism for rotating the eccentric shaft relatively to the revolution shaft while permitting the eccentric shaft to rotate integrally with the revolution shaft.
The additional provision of this revolution radius changing mechanism allows for the tool or the workpiece fixed to the spindle to rotate and revolve while changing its revolution radius, which makes it easy to perform a working operation in which the radius of an object to be worked is changed, for example, a taper boring or recessing operation.
This revolution radius changing mechanism is also constructed to have a differential mechanism for performing differential rotation by means of three rotating elements rotating relatively to one another. Among these three rotating elements, a first rotating element is connected to the transmission mechanism for revolution, a second rotating element is connected to the eccentric shaft and a third rotating element is connected to a motor for changing revolution radius fixed to the base portion.
Due to this construction, a mechanism for rotating the spindle, a mechanism for revolving the spindle and a mechanism for changing radius of revolution can operate independently form one another, and a heavy member such as a motor is not carried in a circular motion. This makes it possible to revolve the spindle at a higher speed and to freely set a ratio of rotation number of the spindle to the revolution number of the same accordingly and further to voluntarily change the revolution radius of the spindle during its revolution.
In a machining apparatus provided with the aforementioned eccentric shaft, the spindle is balanced so that the center of gravity is aligned with its center axis, the eccentric shaft having the spindle at its eccentric position is balanced so that the center of gravity is aligned with its center axis, and the revolution shaft having the eccentric shaft fitted thereon holding the spindle rotatably is balanced so that the center of gravity is aligned with its center axis.
With this construction, even when the revolution radius of the spindle is changed by rotating the eccentric shaft, the position of the center of gravity of the entire working apparatus does not change substantially.
As a result, even when the revolution number of the spindle is increased, no vibration occurs. This makes it possible to increase the revolution speed of the spindle, that is, the tool or the workpiece without causing degradation in the working accuracy and increasing the load on the tool.
In the machining method of the invention, the tool and the workpiece are rotated relatively to each other, the amount of working of the tool per unit time is determined on the basis of at least one of a predetermined maximum sectional area to be worked and cutting speed. On the basis of the amount of working per unit time, there is determined a ratio of the rotation number of the tool or the workpiece to the revolution number thereof.
The tool or the workpiece is rotated and revolved so as to satisfy this ratio, thereby working the workpiece.
According to the method of the invention, therefore, it is possible to set a rotation number and revolution number which have excellent working efficiency without shortening the life of the tool and increasing load on the tool, and thus the efficiency of the working operation can be improved.
In the method of the invention, moreover, the tool and the workpiece are brought into contact with each other while being rotated and revolved. The ratio of rotation number of the tool to the revolution number of the same is set to a value below 37, and the workpiece is intermittently cut by the cutting blade mounted on the tool.
According to the method of the invention, a relative feed between the tool and the workpiece is increased in the intermittent cutting operation in which the tool is rotated. For this reason, cutting amount or working efficiency is not decreased even if the cutting width per blade is reduced. In other words, as the cutting width per blade is reduced without degradation of working efficiency, heat generation, cutting resistance and impact force can be reduced, thereby improving the life of the tool. By increasing the cutting amount per blade within the tool life, moreover, the working efficiency can be improved.
In another method of the invention, the workpiece is cut by means of the blade while one of the tool having the blade and the workpiece is rotated and revolved. Of the cutting speeds at which the workpiece is cut by the blade, the ratio of cutting speed established by the revolution of the tool or the workpiece is set to not less than 7% to cut the workpiece intermittently.
According to the method of the invention, the relative feed between the tool and the workpiece can be increased, so that the load applied to the tool can be decreased according to the reduction of the cutting width per blade to improve the tool life and to improve the working efficiency by increasing the cutting amount within the life of the tool.
In the still another machining method of the invention, the machining operation is performed by rotating and revolving the tool or the workpiece. The ratio of the rotation number to the revolution number is changed on the basis of surface roughness of worked surface formed on the workpiece to perform the aforementioned working operation.
According to the machining method of the invention, various working operations such as rough work or finish work can be performed by means of one apparatus or tool, resulting in the reduction in not only cost of installation but also in number of working steps to raise the working efficiency to a higher level.
In still another method of the invention employing the aforementioned machining apparatus, the machining operation is performed by moving the tool or the workpiece backwards and forwards relatively in the axial direction while rotating and revolving the tool or workpiece. Simultaneously, the ratio of rotation number to revolution number is raised between a working step for moving the tool or the workpiece forwards in the axial direction and working step for moving the tool or the workpiece backwards in the axial direction.
According to this machining method, therefore, the rough work wherein the ratio of rotation number to revolution number is decreased and the finish work wherein this ratio is increased can be performed while the tool or the workpiece moves backwards and forwards in the axial direction. As a result, a total working time can be shortened to improve the productivity.